Treatment method using zinc phosphate-free treatment agent that includes cationic urethane resin, and treated automobile component

ABSTRACT

Provided is a coating pre-treatment method that does not limit the coating method, has a small impact on the environment, and can ensure good post-coating corrosion resistance in a hot-rolled steel sheet. A coating pre-treatment method in which a hot-rolled steel sheet is treated with a chemical conversion treatment agent to form a chemical conversion coating film, wherein the chemical conversion treatment agent includes at least one type (A) of element selected from the group consisting of zirconium, titanium, and hafnium, at least one type (B) of substance selected from the group consisting of amino group-including silane coupling agents, hydrolysates thereof, and polymers thereof, fluorine (C), and a cationic urethane resin (D), and the content of (A) is 20-600 mass ppm in total in terms of metals, and the pH is 3.5-5.5.

TECHNICAL FIELD

The present invention relates to a pre-coating treatment method and a hot-rolled steel sheet.

BACKGROUND ART

Chemical conversion treatment is usually performed for the purpose of improving properties such as corrosion resistance and coating adhesiveness when a surface of a metal material is subjected to cation electrodeposition coating, powder coating, or the like. Chromate treatment is commonly used for chemical conversion in view of its capability of further improving adhesion and corrosion resistance of a coated film. In recent years, the hazardous properties of chromium, however, have been noted, and thus there have been demands for developing a chemical conversion treatment agent which does not contain chromium. As such chemical conversion, widely performed is zinc phosphate treatment.

However, zinc phosphate-based treatment agents contain high concentrations of metal ions and acids, and are highly reactive. This may result in poor waste-treatment economy and poor workability. Further, when a metal surface is treated with a zinc phosphate-based treatment agent, water-insoluble salts may be generated and deposited as precipitates. These precipitates are generally referred to sludge. The removal and disposal of such sludge may add undesirable costs and other problems. Further, phosphate ions may be responsible for increased environmental burden due to eutrophication, and may require additional efforts for waste treatment. Therefore, use of phosphate ions is preferably avoided. Moreover, the treatment of a metal surface with a zinc phosphate-based treatment agent requires surface conditioning. This, disadvantageously, may result in a prolonged process.

As metal-surface treatment agent other than such a zinc phosphate-based treatment agent or a chromate chemical conversion treatment agent, known is a metal-surface treatment agent including a zirconium compound. Such a metal-surface treatment agent including a zirconium compound has a superior property as compared with a zinc phosphate-based chemical conversion treatment agent as described above in that the generation of sludge can be prevented.

Unfortunately, a chemical conversion film obtained by a metal-surface treatment agent including a zirconium compound shows poor adhesiveness, in particular with a coated film obtained by cation electrodeposition coating, and is less often used as a pre-treatment step of cation electrodeposition coating. In such a metal-surface treatment agent including a zirconium compound, a component such as phosphate ions may be used in combination for improving adhesiveness and corrosion resistance. However, when phosphate ions are used in combination, the aforementioned problems such as eutrophication may occur. Moreover, an iron-based base material treated with such a metal-surface treatment agent may have a problem in that neither sufficient coating adhesiveness nor post-coating corrosion resistance can be obtained.

A non-chromate metal-surface treatment agent is also known which includes a zirconium compound and an amino group-containing silane coupling agent. However, surface treatment with such a non-chromate metal-surface treatment agent as an application-type treatment agent used in the field of so-called coil coating is not comparable with post-treatment water-washing. Further, such a non-chromate metal-surface treatment agent is not intended for a target workpiece having a complicated shape.

Furthermore, for an article, such as an automobile body and parts, composed of a metal material such as iron, zinc, and aluminum, the entire metal surface may need to be treated in a single treatment. Accordingly, a pre-coating treatment method is desired to be developed, by which chemical conversion can be performed without causing any problem even in such a case. Meanwhile, a pre-coating treatment method is also desired to be developed, by which chemical conversion can be performed without causing the aforementioned problems even in coating other than cation electrodeposition coating using a powder coating material, a solvent coating material, a water-based paint, and the like.

In an attempt to solve the above problems, a pre-coating treatment method is known, the method involving treating a target workpiece with a chemical conversion treatment agent including at least one selected from the group consisting of zirconium, titanium, and hafnium; fluorine; and at least one selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, polymers thereof to form a chemical conversion film (for example, see Patent Document 1 below).

The above pre-coating treatment method is compatible with any common coating methods, and can provide similar adhesiveness and post-coating corrosion resistance as a case where a zinc phosphate-based chemical conversion treatment agent is used. Nonetheless, it has been difficult to obtain sufficient post-coating corrosion resistance when applied to a hot-rolled steel sheet having a surface on which an oxide film is formed. Such a hot-rolled steel sheet is used in suspension related parts of automobiles and the like.

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-218070

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention is made in view of the above circumstances. An object of the present invention is to provide a pre-coating treatment method which may cause less environmental burden, and can ensure good post-coating corrosion resistance for hot-rolled steel sheets.

Means for Solving the Problems

The present invention relates to a pre-coating treatment method, including: treating a hot-rolled steel sheet with a chemical conversion treatment agent to form a chemical conversion film, wherein the chemical conversion treatment agent includes at least one (A) selected from the group consisting of zirconium, titanium, and hafnium; at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof; fluorine (C); a cationic urethane resin (D), and the total content of (A) is 20 to 600 ppm by mass in terms of metal, and pH is 3.5 to 5.5.

Further, it is preferred that a total of 5 to 1000 ppm by mass of (B) in terms of the solid content concentration is included, and 5 to 1000 ppm by mass of (D) in terms of the solid content concentration is included, and the solid content mass ratio ((B)/(D)) of (B) to (D) is 0.005 to 200.

Moreover, it is preferred that the above chemical conversion treatment agent further contains at least one adhesiveness and corrosion resistance-conferring agent selected from the group consisting of magnesium ions, zinc ions, calcium ions, aluminum ions, gallium ions, indium ions, and copper ions.

The present invention also relates to a hot-rolled steel sheet treated by the above pre-coating treatment method.

Effects of the Invention

The present invention can provide a pre-coating treatment method which is compatible with any common coating method, and may cause less environmental burden, and can ensure good post-coating corrosion resistance for hot-rolled steel sheets.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Below, the embodiments of the present invention will be described. It is noted that the present invention shall not be limited to the following embodiments.

The pre-coating treatment method according to the present embodiment can form a chemical conversion film on a surface of a hot-rolled steel plate as a target workpiece (hereinafter referred to as a “hot-rolled steel sheet”) to ensure preferred post-coating corrosion resistance. There is no particular limitation for a hot-rolled steel sheet as a target workpiece, and a wide spectrum of materials from common hot-rolled steel sheets to specialty steel can be treated. These treated hot-rolled steel sheets are widely used as suspension related parts of automobiles and the like. An oxide film is formed on a surface of a hot-rolled steel sheet as described below, and thus formation of a uniform chemical conversion film on that surface may be difficult. Nonetheless, the pre-coating treatment method according to the present embodiment can form a uniform chemical conversion film on a surface of a hot-rolled steel sheet. Consequently, this can ensure good post-coating corrosion resistance of the treated hot-rolled steel sheet.

The pre-coating treatment method according to the present embodiment is a method of treating a hot-rolled steel sheet, the method including forming a chemical conversion film on a surface of the hot-rolled steel sheet with a chemical conversion treatment agent. The chemical conversion treatment agent according to the present embodiment includes at least one (A) selected from the group consisting of zirconium, titanium, and hafnium; at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof; fluorine (C); and a cationic urethane resin (D). The chemical conversion treatment agent according to the present embodiment is substantially free of phosphate ions and hazardous heavy metal ions, but can form a chemical conversion film having sufficient post-coating corrosion resistance even on a surface of a hot-rolled steel sheet.

The at least one (A) selected from the group consisting of zirconium, titanium, and hafnium corresponds to a component for forming a chemical conversion film. Formation of a chemical conversion film including at least one selected from the group consisting of zirconium, titanium, and hafnium on a base material can improve corrosion resistance and abrasion resistance of the base material, and further can enhance adhesiveness with a coated film.

For example, when a hot-rolled steel sheet is surface treated with a chemical conversion treatment agent containing zirconium, iron ions which are eluted into the chemical conversion treatment agent due to a dissolution reaction of metal may extract fluorine from ZrF₆ ²⁻, or an interface pH may be increased. These may result in generation of hydroxides or oxides of zirconium. These hydroxides or oxides of zirconium are thought to be deposited on a surface of a base material. As described above, the chemical conversion treatment agent according to the present embodiment, which is a reactive chemical conversion treatment agent, can be used even for dipping treatment of a hot-rolled steel sheet having a complicated shape. Moreover, surface treatment performed with the above chemical conversion treatment agent can produce a chemical conversion film adhering firmly on a hot-rolled steel sheet by virtue of a chemical reaction. This also can allow post-treatment water-washing to be performed.

There is no particular limitation for a source of the above zirconium, but examples of the source include, for example, alkali metal fluorozirconate such as K₂ZrF₆; fluorozirconate such as (NH₄)₂ZrF₆; soluble fluorozirconate such as fluorozirconate acid such as H₂ZrF₆; zirconium fluoride; zirconium oxide; and the like.

There is no particular limitation for a source of the above titanium, but examples of the source include, for example, fluorotitanate such as alkali metal fluorotitanate, (NH₄)₂TiF₆; soluble fluorotitanate such as fluorotitanate acid such as H₂TiF₆; titanium fluoride; titanium oxide; and the like.

There is no particular limitation for a source of the above hafnium, but examples of the source include, for example, fluorohafnate acid such as H₂HfF₆; hafnium fluoride; and the like. A source of the at least one selected from the group consisting of zirconium, titanium, and hafnium is preferably a compound having at least one selected from the group consisting of ZrF₆ ²⁻, TiF₆ ²⁻, and HfF₆ ²⁻ in view of high film-forming capability.

The total content of the at least one selected from the group consisting of zirconium, titanium, and hafnium included in the chemical conversion treatment agent according to the present embodiment is within a range between a lower limit of 20 ppm by mass and an upper limit of 600 ppm by mass in terms of metal. When the amount is less than 20 ppm by mass, the resulting chemical conversion film may have insufficient performance. On the other hand an amount of more than 600 ppm by mass can not provide additional effects, and is thus economically disadvantageous. The above lower limit is more preferably 100 ppm by mass. The above upper limit is more preferably 500 ppm by mass, and even more preferably 300 ppm by mass.

The at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof is a compound having at least one amino group in a molecule thereof and also having a siloxane bond. The above at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof can interact with both a chemical conversion film and a coated film. This can improve adhesiveness between them.

This effect can be obtained presumably because a group which can undergo hydrolysis to produce silanol is hydrolyzed and adsorbed on a surface of a metal base material via hydrogen bond, and an amino group can act to enhance adhesiveness between a chemical conversion film and a metal base material. As described above, the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof is thought to act on both a metal base material and a coated film to show an effect of improving mutual adhesiveness.

There is no particular limitation for the above amino group-containing silane coupling agent, but examples thereof can include, for example, publicly known silane coupling agents such as N-2(aminoethyl)3-aminopropylmethyldimethoxysilane, N-2(aminoethyl)3-aminopropyltrimethoxysilane, N-2(aminoethyl)3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, N,N-bis[(3-(trimethoxysilyl)propyl)]ethylenediamine, and the like. Commercially available amino group-containing silane coupling agents KBM-602, KBM-603, KBE-603, KBM-903, KBE-9103, KBM-573 (Shin-Etsu Chemical Co., Ltd.), XS1003 (Chisso Corp.), and the like may also be used.

Hydrolysates of the above amino group-containing silane coupling agent can be prepared by conventionally known methods, for example, by a method including dissolving the above amino group-containing silane coupling agent in ion-exchanged water, and adjusting it to be acidic with any acid, and the like. As a hydrolysate of the above amino group-containing silane coupling agent, a commercially available product such as KBP-90 (Shin-Etsu Chemical Co., Ltd., Active ingredient: 32%) may also be used.

There is no particular limitation for a polymer of the above amino group-containing silane coupling agent, but examples thereof can include, for example, commercially available products such as Sila-Ace S-330 (γ-aminopropyltriethoxysilane; Chisso Corp.), Sila-Ace S-320 (N-(2-aminoethyl)-3-aminopropyltrimethoxysilane; Chisso Corp.).

The total blending amount of the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof in the chemical conversion treatment agent according to the present embodiment is preferably within a range between a lower limit of 5 ppm by mass and an upper limit of 1000 ppm by mass in terms of the solid content concentration. An amount of less than 5 ppm by mass can not provide sufficient coating adhesiveness. An amount of more than 1000 ppm by mass can not provide additional effects, and is thus economically disadvantageous. The above lower limit is more preferably 100 ppm by mass, and even more preferably 200 ppm by mass. The above upper limit is more preferably 400 ppm by masses.

Fluorine (C) can serve as an etching agent for a base material. There is no particular limitation for a source of fluorine (C), but examples of the source can include, for example, fluorides such as hydrofluoric acid, ammonium fluoride, fluoroboric acid, ammonium hydrogen fluoride, sodium fluoride, and sodium hydrogenfluoride. Further, complex fluorides include, for example, hexafluorosilicate, and specific examples thereof can include hydrosilicofluoric acid, zinc hydrofluorosilicate, manganese hydrofluorosilicate, magnesium hydrofluorosilicate, nickel hydrofluorosilicate, iron hydrofluorosilicate, calcium hydrofluorosilicate, and the like.

The cationic urethane resin (D) will form a uniform chemical conversion film on a surface of a hot-rolled steel sheet as a target workpiece. The cationic urethane resin (D) has a cationic functional group. Cationic functional groups include, for example, an amino group, an ammonium group, a methylamino group, an ethylamino group, a dimethylamino group, a diethylamino group, a trimethylamino group, a triethylamino group, and the like. Among these, prepared is a quaternary ammonium group. Moreover, there is no particular limitation for a polyol, isocyanate components of a urethane resin of the cationic urethane resin (D), and a method of polymerization, but conventionally known components and methods may be used. As the cationic urethane resin (D), the followings may be used: for example, commercially available products such as F2667D (DKS Co. Ltd., Effective concentration: 25%), Superflex 620 (DKS Co. Ltd., Effective concentration: 30%), and Superflex 650 (DKS Co. Ltd., Effective concentration: 26%).

Inclusion of the cationic urethane resin (D) alone to a chemical conversion treatment agent can not provide preferred effects such as post-coating corrosion resistance. However, when it is included in a chemical conversion treatment agent in combination with the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof, a uniform chemical conversion film can be formed on a surface of a hot-rolled steel sheet as a target workpiece, ensuring a preferred post-coating anticorrosion properties of the hot-rolled steel sheet. Further, the cationic urethane resin (D) does not undergo a competing reaction with the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof, and thus may be preferably used without inhibiting the functionality of the amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof (B).

The blending amount of the cationic urethane resin (D) in the chemical conversion treatment agent according to the present embodiment is preferably within a range between a lower limit of 5 ppm by mass and an upper limit of 1000 ppm by mass in terms of the solid content concentration. An amount of less than 5 ppm by mass can not provide sufficient coating adhesiveness. An amount of more than 1000 ppm by mass can not provide additional effects, and is thus economically disadvantageous. The above lower limit is more preferably 100 ppm by mass, and even more preferably 200 ppm by mass. The above upper limit is more preferably 400 ppm by mass.

In the chemical conversion treatment agent according to the present embodiment, the mass ratio ((B)/(D)) of the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof to the cationic urethane resin (D) is preferably 0.005 to 200. A mass ratio ((B)/(D)) falling within the above range can provide preferred corrosion resistance of a hot-rolled steel sheet having a chemical conversion film formed thereon. The mass ratio ((B)/(D)) is more preferably 0.05 to 20, and even more preferably 0.5 to 2.

Preferably, the chemical conversion treatment agent according to the present embodiment is substantially free of phosphate ions. The phrase “substantially free of phosphate ions” means that phosphate ions may be included in an amount such that they do not function as a component of a chemical conversion treatment agent. The chemical conversion treatment agent used in the present embodiment is substantially free of phosphate ions. Therefore, essentially no phosphorus is used which is potentially responsible for increased environmental burden. Further, generation of sludge such as iron phosphate and zinc phosphate can be prevented, which otherwise may be generated when a zinc phosphate-based treatment agent is used.

The chemical conversion treatment agent according to the present embodiment has a pH falling within a range between a lower limit of 3.5 and an upper limit of 5.5. A pH of lower than 3.5 may result in excessive etching, and a sufficient film can not be formed. A pH of more than 5.5 may result in insufficient etching, and can not provide a good film. The above lower limit is preferably 3.8, and more preferably 4.0. The above upper limit is preferably 4.7, and more preferably 4.5. In order to adjust a pH of the chemical conversion treatment agent according to the present embodiment, an acidic compound such as nitric acid and sulfuric acid and a basic compound such as sodium hydroxide, potassium hydroxide, and ammonia may be used.

Preferably, the chemical conversion treatment agent according to the present embodiment further includes at least one selected from the group consisting of magnesium ions, zinc ions, calcium ions, aluminum ions, gallium ions, indium ions, and copper ions as an adhesiveness and corrosion resistance-conferring agent. Inclusion of the above adhesiveness and corrosion resistance-conferring agent can provide a chemical conversion film having better adhesiveness and corrosion resistance.

The content of the above at least one selected from the group consisting of magnesium ions, zinc ions, calcium ions, aluminum ions, gallium ions, indium ions, and copper ions is preferably within a range between a lower limit of 1 ppm by mass and an upper limit of 5000 ppm by mass. When the above content is less than the above lower limit, sufficient effects can not be obtained. This is not preferred. When the above content is more than the above upper limit, additional effects can not be obtained. This is economically disadvantageous, and may also decrease post-coating adhesiveness. The above lower limit is more preferably 25 ppm by mass, and the above upper limit is more preferably 3000 ppm by mass.

The above chemical conversion treatment agent may be used in combination with any component in addition to the above components, if needed. Components which can be used can include silica and the like. It is possible to increase post-coating corrosion resistance by adding such a component.

<Pre-Coating Treatment Method>

There is no particular limitation for chemical conversion in the pre-coating treatment method according to the present embodiment, but it may be performed by contacting a chemical conversion treatment agent with a surface of a hot-rolled steel sheet under common treatment conditions. The treatment temperature upon the above chemical conversion is preferably within a range between a lower limit of 20° C. and an upper limit of 70° C. The above lower limit is more preferably 30° C., and the above upper limit is more preferably 50° C. The chemical conversion time for the above chemical conversion is preferably within a range between a lower limit of 5 seconds and an upper limit of 1200 seconds. The above lower limit is more preferably 30 seconds, and the above upper limit is more preferably 120 seconds. There is no particular limitation for a method of conversion treatment, but examples thereof can include, for example, the dipping method, the spray method, the roll coating method, and the like.

In the pre-coating treatment method according to the present embodiment, it is preferred that a surface of a hot-rolled steel sheet may be subjected to degreasing treatment, post-degreasing water-washing treatment before performing the above chemical conversion, and subjected to post-chemical conversion waster-washing treatment after the above chemical conversion. The above degreasing treatment may be performed in order to remove oils and stains adhering on a surface of a base material, and usually performed by dipping treatment at 30 to 55° C. for about several minutes with a degreaser such as phosphorus-free/nitrogen-free degreasing wash liquid. Preliminary degreasing treatment may be performed, if needed, prior to degreasing treatment.

The above post-degreasing water-washing treatment may be conducted by performing spray treatment using a large amount of wash water once or more times in order to wash out a degreaser with water after degreasing treatment. The above post-chemical conversion water-washing treatment may be performed once or more times in order to avoid negative effects on adhesiveness, corrosion resistance, and the like after various subsequent coatings. In that case, the final water-washing is properly performed with pure water. In this post-chemical conversion water-washing treatment, water washing may be performed by either one of spray water-washing or dip water-washing or in combination of these. After the above post-chemical conversion water-washing treatment, drying may be performed in accordance with a known method, if needed, and then various coatings may be applied.

The pre-coating treatment method according to the present embodiment does not require surface conditioning treatment which is required in a conventionally used practical method involving treatment with a zinc phosphate-based chemical conversion treatment agent. This enables chemical conversion of a hot-rolled steel sheet to be performed in fewer steps.

<Hot-Rolled Steel Sheet>

A hot-rolled steel sheet according to the present embodiment has at least one surface on which a chemical conversion film is formed by the pre-coating treatment method according to the present embodiment. There is no particular limitation for the hot-rolled steel sheet according to the present embodiment, and a wide spectrum of materials from common hot-rolled steel sheets to specialty steel can be treated.

A hot-rolled steel sheet may be subjected to rolling at a temperature region of above 800° C. This enables a thick oxide film (a scale) with several μm to tens of μm to be formed on a surface of the steel sheet. Such an oxide film may be removed by performing treatment such as acid wash before use. However, an oxide film may be again generated by heat treatment such as quenching and tempering after processing such as pressing. Further an oxide film itself has anticorrosion properties. In view of these, a chemical conversion film is preferably formed on an oxide film by chemical conversion. Such an oxide film formed on a hot-rolled steel sheet has a fine surface unevenness, and the oxide film is also of a porous state in which a large number of pores are present. For this reason, it is very difficult to form a uniform chemical conversion film on a hot-rolled steel sheet where an oxide film is formed. An ununiform chemical conversion film formed on a surface may cause different potentials between a coated portion and an uncoated portion, preventing formation of a uniform electrodeposition coated film upon electrodeposition coating. Consequently, a pre-coating treatment method using a conventional chemical conversion treatment agent including zirconium and others can not ensure post-coating corrosion resistance comparable to that in a case where a zinc phosphate-based chemical conversion treatment agent. In contrast, a hot-rolled steel sheet treated by the pre-coating treatment method according to the present embodiment has a surface on which a uniform chemical conversion film is formed. Such a hot-rolled steel sheet on which a uniform chemical conversion film is formed has preferred post-coating corrosion resistance.

The mechanism by which such an effect can be obtained is not clearly understood. Nonetheless, one possibility is that the cationic urethane resin (D) may preferentially cover depressed portions and pores of an oxide film through the interaction between the cationic groups of the cationic urethane resin (D) included in a chemical conversion treatment agent and a surface of a steel sheet.

The film content of the chemical conversion film formed on a surface of a hot-rolled steel sheet according to the present embodiment is preferably within a range between a lower limit of 0.1 mg/m² and an upper limit of 500 mg/m² in terms of the total amount of metal included in a chemical conversion treatment agent. An amount of less than 0.1 mg/m² can not provide a uniform chemical conversion film, and is thus not preferred. An amount of more than 500 mg/m² can not provide additional effects, and is thus economically disadvantageous. The above lower limit is more preferably 5 mg/m², and the above upper limit is more preferably 200 mg/m².

A hot-rolled steel sheet treated by the above pre-coating treatment method may be subjected to laser processing, press working, and the like to obtain a metal member formed and processed depending on various purposes. Alternatively, a pre-formed and processed hot-rolled steel sheet may be subjected to the above pre-coating treatment method. There is no particular limitation for the applications of a metal member according to the present embodiment, but examples of thereof include metal members of automobiles such as a door, a bonnet, a roof, a hood, a fender, a trunk room, and the like. Further, they also include metal members used for motorcycles, buses, bicycles, and the like. A metal member made of a hot-rolled steel sheet treated by the above pre-coating treatment method may preferably be used in those applications as described above in which a high level of post-coating corrosion resistance is required in view of safely and aesthetics.

There is no particular limitation for coating which can be performed on a hot-rolled steel sheet treated by the pre-coating treatment method according to the present embodiment, but coating may be performed with a conventionally known coating material such as a cationic electrodeposition coating material, a solvent coating material, a water-based coating material, and a powder coating material. For example, there is no particular limitation for the above cationic electrodeposition coating material, but a conventionally known cationic electrodeposition coating material including an aminated epoxy resin, aminated acrylic resin, a sulfonated epoxy resin, and the like may be applied. Amount these, a cationic electrodeposition coating material including a resin having a functional group which shows reactivity or compatibility with an amino group is preferred in order to enhance adhesiveness between an electrodeposition coated film and a chemical conversion film, considering that at least one selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof is blended in a chemical conversion treatment agent.

The present invention shall not be limited to the above embodiments. Modifications, improvements, and the like can be made within a scope of the present invention as long as an effect of the present invention can be achieved.

EXAMPLES

Next, the present invention will be described in more detail with reference to Examples, but the present invention shall not be limited to these Examples. It is noted that the term “ppm” as used in Examples and Comparative Examples refers to “ppm by mass.”

Example 1

A commercially available hot-rolled steel plate (SPH 270, Nippon Testpanel Co., Ltd., 70 mm×150 mm×0.8 mm) as a base material was subjected to pre-coating treatment under the following conditions.

(1) Pre-Coating Treatment

Degreasing treatment: Dipping treatment was performed at 40° C. with 2% by mass of “Surfcleaner 53” (a degreaser from Nippon Paint Surf Chemicals Co., Ltd.). Post-degreasing water-washing treatment: Spray treatment was performed with tap water for 30 seconds. Chemical conversion treatment: Zircon hydrofluoric acid and KBM-603 (N-2(aminoethyl)3-aminopropyltrimethoxysilane, Effective concentration: 100%, Shin-Etsu Chemical Co., Ltd.) as an amino group-containing silane coupling agent; and F2667D (DKS Co. Ltd., Effective concentration: 25%) as a cationic urethane resin were used to prepare a chemical conversion treatment agent including zirconium (A) in a concentration of 100 ppm by mass, an amino group-containing silane coupling agent (B) in a concentration of 100 ppm by mass in terms of the solid content, and a cationic urethane resin (D) in a concentration of 100 ppm by mass. Sodium hydroxide was used to adjusted pH to 4. The temperature of the chemical conversion treatment agent was adjusted to 40° C., and a base material was dip-treated for 60 seconds. The film amount in the initial stage of the treatment was 13.4 mg/m².

Post-chemical conversion water-washing treatment: Spray treatment was performed with tap water for 30 seconds. Further, spray treatment was performed with ion-exchanged water for 10 seconds. Then, electrodeposition coating was performed in a wet condition. A cold-rolled steel sheet after water washing was dried at 80° C. for 5 minutes in an electric drying furnace, and then the film amount was analyzed as the total amount of metal contained in a chemical conversion treatment agent with a “ZSX PrimusII” (an X-ray analyzer from Rigaku Corporation).

(2) Coating

A cold-rolled steel plate was treated with a chemical conversion treatment agent at 1 L per m², and then electrodeposition-coated with “Powernics 310” (a cationic electrodeposition coating material from Nipponpaint Industrial Coatings Co., Ltd.) so as to obtain a dry coating thickness of 20 μm, and washed with water, and then heated for baking at 170° C. for 20 minutes to obtain a test plate.

Examples 2 and 3

Test plates were prepared as in Example 1 except that a hot-rolled steel plate (SPH 440, SPH 590 from Nippon testpanel Co., Ltd., 70 mm×150 mm×0.8 mm) was used as a base material.

Examples 4, 5, 8, 10, 19 and 20

Test plates were prepared as in Example 1 except that the concentrations of the silane coupling agent (B) and the cationic urethane resin (D) were varied as shown in Table 1.

Examples 6 and 7

Test plates were prepared as in Example 1 except that Superflex 620 (DKS Co. Ltd., Effective concentration: 30%) or Superflex 650 (DKS Co. Ltd., Effective concentration: 26%) was used as the cationic urethane resin (D) as shown in Table 1.

Examples 9, 11 and 12

As shown in Table 1, test plates were prepared as in Example 1 except that the concentration of zirconium (A) was 100 ppm by mass or 500 ppm by mass, and KBM-903 (3-aminopropyltrimethoxysilane, Effective concentration: 100%, Shin-Etsu Chemical Co., Ltd.) or XS1003 (N,N-bis[3-(trimethoxysilyl)propyl]ethylenediamine, Effective concentration: 50%, Nichibitrading Co., Ltd.), or KBE-903 (3-aminopropyltriethoxysilane, Effective concentration: 100%, Shin-Etsu Chemical Co., Ltd.) was used as the silane coupling agent (B), and the concentrations of the silane coupling agent (B) and the cationic urethane resin (D) were varied as shown in Table 1.

Examples 13 to 18

Test plates were prepared as in Example 1 except that the concentration of zirconium (A) was varied as shown in Table 1, and zinc nitrate (Zn) was used as an adhesiveness and corrosion-resistance conferring agent, and the concentrations of the silane coupling agent (B) and the cationic urethane resin (D) were varied as shown in Table 1.

Comparative Examples 1 to 5

Test plates were prepared as in Example 1 except that the concentrations of the silane coupling agent (B) and the cationic urethane resin (D) were varied as shown in Table 1.

Reference Example 1

A test plate was prepared as in Example 1 except that as shown in Table 1, surface conditioning was performed with Surffine GL1 (Nippon Paint Surf Chemicals Co., Ltd.) at room temperature for 30 seconds after post-degreasing water-washing treatment, and then chemical conversion treatment was performed by dipping treatment using Surfdine SD-5350 (a zinc phosphate-based chemical conversion treatment agent from Nippon Paint Surf Chemicals Co., Ltd.) at 35° C. for 2 minutes instead of using the above chemical conversion treatment agents.

The following evaluation tests were performed for the test plates obtained as described above from Examples 1 to 20, Comparative Examples 1 to 5, and Reference Example 1.

[Secondary Adhesiveness Tests (SDT)]

The resulting test plates were each nicked deep enough to reach an underlying material along two parallel and longitudinal lines, and then dipped under a 5% NaCl aqueous solution at 50° C. for 480 hours. Subsequently, a cut portion was exfoliated off with a tape, and the exfoliation state of a coating material was observed. The exfoliation state was evaluated in accordance with the following evaluation criteria, and an evaluation score of 2 or more was considered as acceptable. The results were shown in Tables 1 and 2.

1: Not exfoliated 2: Somewhat exfoliated 3: Exfoliation width is 3 mm or more

[Salt-Water Spray Tests (SST)]

The resulting test plates were each cross-cut deep enough to reach an underlying material, and continuously sprayed with a 5% NaCl aqueous solution for 240 hours in a salt-water spry test chamber maintained at 35° C. Subsequently, the width of a blister from a cut portion was measured. Those having a blister width comparable to or less than that in a case where a zinc phosphate-based surface treatment agent was used as shown in Reference Example 1 was considered as acceptable. The results were shown in Tables 1 and 2.

[Combined Cyclic Corrosion Tests (CCT)]

The resulting test plates were each cross-cut deep enough to reach an underlying material, and then combined cyclic corrosion tests were performed. Combined tests were performed for 100 cycles by a test method in accordance with JASO M609-91. After the tests, the width of a blister from a cut portion was measured. Those having a blister width comparable to or less than that in a case where a zinc phosphate-based surface treatment agent was used as shown in Reference Example 1 was considered as acceptable. The results were shown in Tables 1 and 2.

TABLE 1 Adhesiveness and Cationic corrosion-resistance Silane coupling urethane Zirconium conferring agent agent(B) resin(D) concen- Concen- Solid content Solid content tration(A) tration concentration concentration (ppm) Types (ppm) Types (ppm) Types (ppm) Examples 1 100 None 0 KBM-603 100 F2667D 100 2 100 None 0 KBM-603 100 100 3 100 None 0 KBM-603 100 100 4 100 None 0 KBM-603 5 5 5 100 None 0 KBM-603 50 50 6 100 None 0 KBM-603 100 Superflex 620 100 7 100 None 0 KBM-603 100 Superflex 650 100 8 100 None 0 KBM-603 5 F2667D 1000 9 100 None 0 KBM-903 1000 5 10 100 None 0 KBM-603 1000 1000 11 500 None 0 XS-1003 100 100 12 500 None 0 KBE-903 1000 1000 13 100 None 500 KBM-603 100 100 14 20 None 500 KBM-603 400 400 15 600 None 500 KBM-603 400 400 16 200 None 500 KBM-603 400 400 17 200 None 500 KBM-603 200 400 18 200 None 500 KBM-603 400 200 19 100 None 0 KBM-603 5 100 20 100 None 0 KBM-603 100 5 Compar- 1 100 None 0 KBM-603 0 F2667D 100 ative 2 100 None 0 KBM-603 0 100 Examples 3 100 None 0 KBM-603 0 100 4 100 None 0 KBM-603 100 0 5 100 None 0 KBM-603 0 5000 Reference 1 Zinc phosphate treatment Example Amount of Base film SST CCT ((B)/(D)) material (mg/m²) Coating SDT (mm) (mm) Examples 1 1 SPH270 17.6 Powernics 1 1.7 5.1 2 1 SPH440 21.8 310 1 2.0 7.2 3 1 SPH590 25.3 1 1.9 9.9 4 1 SPH270 22.2 2 2.2 9.2 5 1 SPH270 16.2 1 1.6 6.5 6 1 SPH270 15.9 1 2.0 7.4 7 1 SPH270 18.4 1 2.2 7.6 8 0.005 SPH270 17.4 1 1.5 5.9 9 200 SPH270 14.6 1 1.8 5.9 10 1 SPH270 11.6 1 1.6 7.0 11 1 SPH270 40.8 1 1.6 5.0 12 1 SPH270 14.2 1 1.8 6.8 13 1 SPH270 17.2 1 1.3 4.9 14 1 SPH270 11.2 1 1.9 7.1 15 1 SPH270 19.6 1 1.9 8.2 16 1 SPH270 19.7 1 1.6 4.5 17 05 SPH270 21.2 1 0.9 5.0 18 2 SPH270 20.2 1 0.9 5.0 19 0.05 SPH270 16.3 2 1.9 8.7 20 20 SPH270 17.8 1 2.2 10.2 Compar- 1 — SFH270 26.4 Powernics 3 2.4 11.9 ative 2 — SPH440 30.3 310 3 2.2 15.3 Examples 3 — SPH590 34.1 3 2.5 14.2 4 — SPH270 21.4 1 2.0 13.5 5 — SPH270 22.1 3 2.0 14.3 Reference 1 Zinc phosphate treatment SPH270 2500 2 2.5 11.3 Example

Comparison of Examples 1 to 20 with Comparative Examples 1 to 3 and 5 shows that the hot-rolled steel sheets treated with the chemical conversion treatment agents from Examples 1 to 20 have superior secondary adhesiveness (SDT) as compared with the hot-rolled steel sheets treated with the chemical conversion treatment agents from Comparative Examples 1 to 3 and 5. These results demonstrate that preferred post-coating corrosion resistance can be conferred on a hot-rolled steel sheet by performing pre-coating treatment of the hot-rolled steel sheet with a chemical conversion treatment agent including the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof. Further, neither the hot-rolled steel sheet treated with the chemical conversion treatment agent from Comparative Example 1 nor the hot-rolled steel sheet treated with the chemical conversion treatment agent from Comparative Example 5 show preferred secondary adhesiveness (SDT). This indicates that an increased content of the cationic urethane resin (D) can not provide preferred results when a chemical conversion treatment agent does not contain the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof, and also indicates that preferred post-coating corrosion resistance can be conferred on a hot-rolled steel sheet by pre-coating treatment of the hot-rolled steel sheet with a chemical conversion treatment agent including the at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof in combination with the cationic urethane resin (D).

Comparison of Examples 1 to 20 with Comparative Example 4 shows that the hot-rolled steel sheets treated with the chemical conversion treatment agents from Examples 1 to 20 have superior results from the combined cyclic corrosion tests (CCT) as compared with the hot-rolled steel sheet treated with the chemical conversion treatment agent from Comparative Example 4. These results demonstrate that preferred post-coating corrosion resistance can be conferred on a hot-rolled steel sheet by pre-coating treatment of the hot-rolled steel sheet with a chemical conversion treatment agent including the cationic urethane resin (D).

Comparison of Examples 1 to 20 with Reference Example 1 shows that the hot-rolled steel sheets treated with the chemical conversion treatment agents from Examples 1 to 20 have comparable or superior results from the salt-water spray tests (SST), the combined cyclic corrosion tests (CCT) as compared with the hot-rolled steel sheet treated with the chemical conversion treatment agent from Reference Example 1. These results indicate that the pre-coating treatment method involving use of the chemical conversion treatment agent according to an embodiment of the present invention can confer comparable or superior post-coating corrosion resistance on a hot-rolled steel sheet as compared with the pre-coating treatment method involving use of a conventional zinc phosphate-based treatment agent. 

1. A pre-coating treatment method, comprising: treating a hot-rolled steel sheet with a chemical conversion treatment agent to form a chemical conversion film, wherein the chemical conversion treatment agent includes at least one (A) selected from the group consisting of zirconium, titanium, and hafnium, at least one (B) selected from the group consisting of an amino group-containing silane coupling agent, hydrolysates thereof, and polymers thereof, fluorine (C), and a cationic urethane resin (D), and wherein the total content of (A) is 20 to 600 ppm by mass in terms of metal, and pH is 3.5 to 5.5.
 2. The pre-coating treatment method according to claim 1, wherein the total content of (B) is 5 to 1000 ppm by mass in terms of a solid content concentration, the content of (D) is 5 to 1000 ppm by mass in terms of a solid content concentration, and the solid content mass ratio ((B)/(D)) of (B) to (D) is 0.005 to
 200. 3. The pre-coating treatment method according to claim 1, wherein the chemical conversion treatment agent further includes at least one adhesiveness and corrosion resistance-conferring agent selected from the group consisting of magnesium ions, zinc ions, calcium ions, aluminum ions, gallium ions, indium ions, and copper ions.
 4. A hot-rolled steel sheet treated with the pre-coating treatment method according to claim
 1. 